From: Mathieu Desnoyers Date: Wed, 24 Nov 2021 19:13:28 +0000 (-0500) Subject: Fix: combined tracing of lttng-ust 2.12/2.13 generates corrupted traces X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=66d0d054fda6eb3c8066b2e1a180231aee23b644;p=lttng-ust.git Fix: combined tracing of lttng-ust 2.12/2.13 generates corrupted traces Observed issue ============== When tracing applications linked against lttng-ust 2.12 and lttng-ust 2.13 in parallel with a lttng-tools 2.13 into the same per-uid buffers, with the "procname" context enabled, babeltrace fails with "Event id NN is outside range" when reading the trace: [14:51:58.717006865] (+5.927872956) x lttng_ust_statedump:start: { cpu_id = 1 }, { procname = "sample-2.13-ust" }, { } [error] Event id 41984 is outside range. [error] Reading event failed. Error printing trace. Cause ===== Inspection of the trace reveals that the layout of the procname context field changed from 17 bytes to 16 bytes between 2.12 and 2.13. This is an issue when applications share a per-uid ring buffer, because context fields are associated with channels, and need to have the same layout across all processes tracing into a given channel. The layout of the procname field described by the trace metadata is that of the first application which happens to register that channel in the session lifetime. Therefore, the procname context field length is part of the LTTng-UST ABI and cannot be changed without breaking the LTTng-UST ABI (bumping LTTNG_UST_ABI_MAJOR_VERSION_OLDEST_COMPATIBLE), which is unwanted between 2.12 and 2.13. Keeping compatibility for combined use by different applications between lttng-ust 2.12 and 2.13 is a required feature for this release, because lttng-ust 2.13 introduces a library ABI break (soname bump). An example scenario leading to this issue: 1) trace created for per-uid buffers, 2) add procname context 3) start tracing 4) Application [a] linked against lttng-ust 2.13 registers the channel to lttng-sessiond, sending its context descriptions with a 16-byte procname context field, 5) Application [b] linked against lttng-ust 2.12 registers the same channel to lttng-sessiond, 6) Application [b] traces an event with the procname context, followed by an event payload with a single "string" field. 7) A trace viewer will observe the procname context, followed by an extra null character, and thus mistakenly consider the event payload to be an empty string. Reading the next event header will fail because the string payload will be expected to contain an event ID. Solution ======== Revert the procname context field size to 17 bytes to stay compatible with lttng-ust 2.12. In an abundance of caution, also revert the size of the lttng_ust_statedump:procname "procname" field to 17, so there won't be duplicated event IDs for this event when applications linked against lttng-ust 2.12 and 2.13 are traced concurrently for the same user ID in per-uid tracing. History ======= This issue was introduced by commit 0db3d6ee9be ("port: fix pthread_setname_np integration") within the 2.13 development cycle. Known drawbacks =============== Applications currently running which are linked against a liblttng-ust 2.13 without this fix should be restarted after upgrading the library to liblttng-ust 2.13 with this fix. Signed-off-by: Mathieu Desnoyers Change-Id: I206086df8b71463c248ca186343baaff5452762b --- diff --git a/src/common/events.h b/src/common/events.h index 338573a9..76062828 100644 --- a/src/common/events.h +++ b/src/common/events.h @@ -18,6 +18,13 @@ #include "common/macros.h" #include "common/ust-context-provider.h" +/* + * The context procname length is part of the LTTng-UST ABI. + * TODO: At the next breaking protocol bump, all users of this macro + * should instead use LTTNG_UST_ABI_PROCNAME_LEN. + */ +#define LTTNG_UST_CONTEXT_PROCNAME_LEN 17 + struct lttng_ust_abi_obj; struct lttng_event_notifier_group; diff --git a/src/lib/lttng-ust/lttng-context-procname.c b/src/lib/lttng-ust/lttng-context-procname.c index 80ba9825..b5bf77be 100644 --- a/src/lib/lttng-ust/lttng-context-procname.c +++ b/src/lib/lttng-ust/lttng-context-procname.c @@ -30,7 +30,7 @@ * be set for a thread before the first event is logged within this * thread. */ -typedef char procname_array[PROCNAME_NESTING_MAX][17]; +typedef char procname_array[PROCNAME_NESTING_MAX][LTTNG_UST_CONTEXT_PROCNAME_LEN]; static DEFINE_URCU_TLS(procname_array, cached_procname); @@ -47,8 +47,8 @@ const char *wrapper_getprocname(void) CMM_STORE_SHARED(URCU_TLS(procname_nesting), nesting + 1); /* Increment nesting before updating cache. */ cmm_barrier(); - lttng_pthread_getname_np(URCU_TLS(cached_procname)[nesting], LTTNG_UST_ABI_PROCNAME_LEN); - URCU_TLS(cached_procname)[nesting][LTTNG_UST_ABI_PROCNAME_LEN - 1] = '\0'; + lttng_pthread_getname_np(URCU_TLS(cached_procname)[nesting], LTTNG_UST_CONTEXT_PROCNAME_LEN); + URCU_TLS(cached_procname)[nesting][LTTNG_UST_CONTEXT_PROCNAME_LEN - 1] = '\0'; /* Decrement nesting after updating cache. */ cmm_barrier(); CMM_STORE_SHARED(URCU_TLS(procname_nesting), nesting); @@ -70,7 +70,7 @@ size_t procname_get_size(void *priv __attribute__((unused)), struct lttng_ust_probe_ctx *probe_ctx __attribute__((unused)), size_t offset __attribute__((unused))) { - return LTTNG_UST_ABI_PROCNAME_LEN; + return LTTNG_UST_CONTEXT_PROCNAME_LEN; } static @@ -82,7 +82,7 @@ void procname_record(void *priv __attribute__((unused)), const char *procname; procname = wrapper_getprocname(); - chan->ops->event_write(ctx, procname, LTTNG_UST_ABI_PROCNAME_LEN, 1); + chan->ops->event_write(ctx, procname, LTTNG_UST_CONTEXT_PROCNAME_LEN, 1); } static @@ -95,7 +95,7 @@ void procname_get_value(void *priv __attribute__((unused)), static const struct lttng_ust_ctx_field *ctx_field = lttng_ust_static_ctx_field( lttng_ust_static_event_field("procname", - lttng_ust_static_type_array_text(LTTNG_UST_ABI_PROCNAME_LEN), + lttng_ust_static_type_array_text(LTTNG_UST_CONTEXT_PROCNAME_LEN), false, false), procname_get_size, procname_record, diff --git a/src/lib/lttng-ust/lttng-ust-comm.c b/src/lib/lttng-ust/lttng-ust-comm.c index 2738959f..b1c35480 100644 --- a/src/lib/lttng-ust/lttng-ust-comm.c +++ b/src/lib/lttng-ust/lttng-ust-comm.c @@ -259,7 +259,7 @@ struct sock_info { int statedump_pending; int initial_statedump_done; /* Keep procname for statedump */ - char procname[LTTNG_UST_ABI_PROCNAME_LEN]; + char procname[LTTNG_UST_CONTEXT_PROCNAME_LEN]; }; /* Socket from app (connect) to session daemon (listen) for communication */ @@ -482,7 +482,7 @@ int setup_global_apps(void) } global_apps.allowed = 1; - lttng_pthread_getname_np(global_apps.procname, LTTNG_UST_ABI_PROCNAME_LEN); + lttng_pthread_getname_np(global_apps.procname, LTTNG_UST_CONTEXT_PROCNAME_LEN); error: return ret; } @@ -528,7 +528,7 @@ int setup_local_apps(void) goto end; } - lttng_pthread_getname_np(local_apps.procname, LTTNG_UST_ABI_PROCNAME_LEN); + lttng_pthread_getname_np(local_apps.procname, LTTNG_UST_CONTEXT_PROCNAME_LEN); end: return ret; } diff --git a/src/lib/lttng-ust/lttng-ust-statedump-provider.h b/src/lib/lttng-ust/lttng-ust-statedump-provider.h index 0cad9ad8..916a3564 100644 --- a/src/lib/lttng-ust/lttng-ust-statedump-provider.h +++ b/src/lib/lttng-ust/lttng-ust-statedump-provider.h @@ -88,7 +88,7 @@ LTTNG_UST_TRACEPOINT_EVENT(lttng_ust_statedump, procname, ), LTTNG_UST_TP_FIELDS( lttng_ust_field_unused(session) - lttng_ust_field_array_text(char, procname, name, LTTNG_UST_ABI_PROCNAME_LEN) + lttng_ust_field_array_text(char, procname, name, LTTNG_UST_CONTEXT_PROCNAME_LEN) ) )